The present invention relates to a method for measuring the concentration of an adhesive on the surface of a coating layer coated on a support such as coated paper.
Paper is coated on the surface thereof with various types of pigments to improve its smoothness, brightness, ink receptivity, gloss and opacity. To fabricate such coated paper, adhesives are used to bind pigment particles to each other and to keep the particles on the surface of the base stock. The adhesive typically contains coating slurry together with pigment such as clay or calcium carbonate, and is applied to the surface of the base stock.
When coatings are applied to porous materials such as paper, the adhesive might, however, migrate on the surface of the coating layer or toward the base paper, sometimes into the base paper, a phenomenon known as binder migration, especially during, for example, a drying step. When binder migration occurs, the concentration of adhesive on the surface correspondingly increases, adversely affecting the printability of the paper. For example, binder migration may cause improper inking (print mottle). When the concentration of the adhesive is mottled on the surface of the coating layer, an undesirable effect is produced in that the ink is mottled when offset printing is being conducted. This adverse influence probably occurs because the adhesive itself has no affinity to the printing ink.
In view of the problems discussed above, it is very important to know the concentration of the adhesive on the surface of the coating layer and to know the surface concentration distribution to predict the printability of the coated paper. Thus, the present invention is directed to a technique for accurately measuring the surface concentration of the adhesive and for accurately measuring the surface concentration distribution. Known methods for measuring the concentration of the adhesive on the surface of a coating layer include infrared spectroscopic analysis, analysis with an X-ray microanalyzer and the like.
The infrared spectroscopic analysis is simple but is subject to large errors since the sensitivity of the adhesive is much lower than that of the pigment. The X-ray microanalyzer has, on the other hand, high sensitivity and can measure the distribution of the adhesive on the surface of the coating layer, but necessitates prior treatment with a dye, such as osmium or bromine.
The inventor of the present invention has discovered a correlation between the adhesive concentration on the surface of the coating layer of the coated paper and the ultraviolet ray absorbance and has discovered that styrene-butadiene latex (SB latex) exhibits special absorbance in the ultraviolet ray zone and that the absorbance significantly differs from that of the pigment. The inventor of the present invention has applied the ultraviolet ray absorbance of SB latex to the measurement of the concentration of the adhesive.
However, reflection measurement of light scattered on a sample surface must be employed in the measurement of the concentration of adhesive on the surface of the coating layer. Thus, when light is scattered on the surface of the coating layer because of gloss, the measurement is affected by the scattered light and the concentration of the adhesive cannot be accurately measured.